VILNIUS — Nestled between highlands and an icy coast in northeastern Europe, Lithuania resisted Christianity for longer than any other nation on the continent. The Lithuanians finally converted in 1387 as part of a grand geostrategic bargain — then used the deal to expand their influence over thousands of miles of territory between the Baltic Sea and the Black Sea in the south. Centuries later, Lithuania became the first state to leave the crumbling Soviet Union.